Electrical heating element



Patented Sept. ll, 1%23.

mea er PERCY A, E. ARMSTRONG, OF LOUDONVILLE, NEW YORK.

ELECTRICAL HEATING ELEMENT.

NoIDrawing. Application filed December 13, 1919, Serial No. 344,737. Renewed November 24, 1922.

To all whom it m any concern:

Be it known that I, PERCY A. E. ARM- s'rnoNo, a subject of the King of Great Britain, and a resident of Loudonville, county of Albany, and State of New York, have invented certain new and useful Improvements in Electrical Heating Elements, of which the following is a specification.

My invention relates to electrical heating elements of alloy steel containing essentially iron, chromium, silicon, aluminum and carbon.

Heating elements in accordance with my invention may contain chromium from about 3% to 30%, silicon from about .5% to about 5%, aluminum fromabout .4% to about 4%, carbon from about .05% to about 1.5% and preferably under about and the principal part of the remainder iron.

More or less of the metalloids, such as sulphur and phosphorus, may be present, but thesame are preferably kept low. Manganese may be present in the usual proportions encountered in steel making, say

up to about .85%, but manganese may be dispensed with, if desired.

Relatively cheap electrical heating elements, which are not required to withstand temperatures over about 1300 F may contain as little as about 3% of chromium, about. 1% of silicon, about .5% of aluminum, carbon preferably from about .0'5% to about .5% and the principal part of the remainder iron. This analysis represents about the minimum proportions of the different ma terials to give a fair amount of electrical resistance and a. good degree of resistance to hot oxidation.

A good example of an element to withstand temperatures up to about 1700 F. is, chromium about 10%, silicon about 3%, aluminum about 3% and carbon about 05% to about 50%.

Another element which gives good results at about the last named temperatures con tains chromium about 10%,v silicon about 1%}%, aluminum about 1%, and carbon as before. With the higher contentof silicon and aluminum, taken together, increased specific resistance is obtained. With the alloy last described, and having he content of silicon and aluminum relatively low, a larger wire can be employed which is better adapted for more rugged applications where size and strength are important requirements.

A good example of an element to withstand higher temperatures, say up to about 2000 F. to 2200 F., contains chromium about 16%, silicon about 3%, aluminum about 45% and carbon about 05% to about .50 Elements of this analysis having about 24% of carbon Were found to have a specific resistance of 143 microhms per cubic centimeter and desirably long life.

One of the beneficial effects of aluminum used in conjunction with silicon is that high specific resistance can be obtained without the use of an unduly large quantity of alloying constituents, making a material which can be mechanically worked to good advantage. Aluminum when used as described, appears to have the valuable property of either holding the gases in solution or taking the oxygen out of the alloy steel. WVith too great a percentage of aluminum, however, the material is both hot and cold short, but with relatively low amounts of aluminum added, about as stated, the combination of iron, chromium, silicon and carbon is materially improved and better results obtained. Under these circumstances I have found that in my electrical heating elements of alloy steel, silicon and aluminum are not interchangeable, one for the other, but must be used together to get good results. The eflect of small quantities of aluminum is most marked in the case of elements for withstanding high temperatures given above. Here the aluminum shown in the analysis of the finished material is but 45%, yet with the same chromium, silicon and carbon, but without aluminum, nowhere near as satisfactory results are obtained. 'With the use of aluminum as described, a very thin and resistant scale'appears to be quickly formed on the surface of the element when subjected to heat and to oxidizing agencies and this skin coating effectively resists further oxidation and scaling in use at high temperatures.

I have found that by using carbon under 1.5% and preferabl under .5%, I can use a much cheaper gradia of material than Without the use of carbon. The carbon has a desirable property of refining the grain and preventin undue grain growth under conditions 0% use, it appears to increase the specific resistance to some extent, it causes the material to flow or conform well to groove shapes in rolling, hastens solidification of the melt, and cuts down the plastic range enabling the elimination of hollow or spongy centers, improves the annealing properties, and imparts a degree of stiffness to the heating elements which is particularly desirable in lateral applications.

Where the elements are made by hot work ing, as by rolling hot, the carbon content can be increased up to about 1.5%, but for elements made by cold Working as by cold drawing into wire the carbon content is preferably under 50% and may be as low as 05%.

Proportions given herein are by weight. It will be understood that more or less variation from the proportio'ns' stated can be resorted to, if desired, without departing from my invention or sacrificing the advantages thereof.

1 claim:

1. An electrical heating element made of alloy steel containing chromium about 3% to about silicon about .5% to about 5%, aluminum about 4% to about 4%, carbon about .05% to about 1.5%, and the principal portion of the remainder; iron.

2. An electrical heating element made of alloy steel containing chromium about 10% to about 20%, aluminum about 4% to about 4%, silicon about 1% to about 5% and more silicon than aluminum, carbon about .05% to about .50% and the principal part of the remainder iron.

3. An electrical heating element of alloy steel containing chromium about 16%, sili con about 3%, aluminum about .45%, carbon under 50%, and the principal portion of the remainder iron.

In testimony that I claim the foregoing, 1 hereto set my hand, this 29th day of November, 1919.

I PERCY A. E. ARMSTRONG. 

